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Williamson played college football for Chicago's Northwestern University during the late 1950s. This was followed by a seven-year professional career in the NFL, beginning in 1960 when he was signed on with the Pittsburgh Steelers. The following year, he joined the Oakland Raiders, with whom he played four seasons. He then played three seasons with the Kansas City Chiefs. It was during this time he became known as "The Hammer", and was a starter in the very first Super Bowl.

He moved into acting in the late 1960s; his appearance on Star Trek was one of his earliest endeavors in the field. Another early television appearance was a 1969 episode of the short-lived series The Outsider with fellow TOS guest star Michael Strong.

Williamson made his feature film debut playing Captain Oliver "Spearchucker" Jones in Robert Altman's irreverent 1970 comedy M*A*S*H. This film also marked the film debut of future Trek alum John Schuck and co-starred Rene Auberjonois and Sally Kellerman.

From 1970 through 1971, Williamson made regular appearances on the television sitcom Julia, playing the title character's love interest, Steve Bruce. He was a replacement for Paul Winfield, who played Julia's previous boyfriend, Paul Cameron. Williamson's co-stars on this series included fellow TOS guest actor Don Marshall.

For the remainder of the 1970s, Williamson was a fixture of action films in the "blaxploitation" genre. He also produced, directed and written a number of these films. He played the title role of films like The Legend of Nigger Charley (1972) and its 1973 sequel, The Soul of Nigger Charley, Black Caesar (1973), Boss Nigger (1975, which he also produced), and Mean Johnny Barrows (1976, with Anthony Caruso), the latter of which he also directed and produced. In the 1974 blaxploitation film Three the Hard Way, Williams co-starred with Jay Robinson, Corbin Bernsen, and Irene Tsu.

Throughout the 1980s, Williamson worked on a number of Italian B-movies, mostly science fiction action films, including The New Barbarians and Bronx Warriors (both 1982) and The New Gladiators (1984). He also continued directing, producing, and starring in the occasional blaxploitation film, such as 1983's The Big Score (co-starring Ed Lauter).

Williamson became a regular on the short-lived TV series Half Nelson, which ran for seven episodes in 1985. In addition, he co-starred with Star Trek: The Motion Picture actress Persis Khambatta in the Italian science fiction film Warrior of the Lost World (1983) and in the 1988 direct-to-video thriller Deadly Intent.